Ind. Eng. Chem. Res. 2002, 41, 691-696
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Kinetics of Aqueous-Phase Hydrogenation of Lactic Acid to Propylene Glycol Zhigang Zhang,† James E. Jackson,‡ and Dennis J. Miller*,† Departments of Chemical Engineering and Chemistry, Michigan State University, East Lansing, Michigan 48824
The kinetics of aqueous-phase hydrogenation of lactic acid to propylene glycol over a 5 wt % Ru/carbon catalyst have been characterized in a stirred batch reactor. A thorough analysis of mass-transfer resistances based on measurements of hydrogen solubility and gas-liquid masstransfer coefficients, application of correlations in the literature, and intraparticle diffusion calculations show that mass-transfer resistances are negligible at the temperatures (403-423 K) and hydrogen pressures (6.8-13.6 MPa) studied. A Langmuir-Hinshelwood (L-H) model is proposed and used to fit lactic acid conversion kinetics. The kinetic model provides insight into the catalytic reaction mechanism and forms the basis for design and further investigation of the aqueous-phase hydrogenation. I. Introduction In a recent paper,1 we reported the aqueous-phase hydrogenation of 2-hydroxypropanoic acid (lactic acid or LA) to 1,2-propanediol (propylene glycol or PG) over supported metal catalysts. Results of catalyst screening, optimization of reaction conditions, and evaluation of the effects of residual salts from lactate fermentation were presented. Yields of PG as high as 92% of the theoretical were achieved over a 5 wt % Ru on activated carbon catalyst at 150 °C and 14.5 MPa H2 in a stirred batch reactor. These initial studies demonstrated higher sustained PG yields at milder reaction conditions than prior studies that used either lactate esters2-4 or aqueous-phase free lactic acid5-8 as the starting material. The catalytic hydrogenation of lactic acid to propylene glycol is a three-phase reaction involving hydrogen gas, aqueous solution of lactic acid and product propylene glycol, and solid catalyst. Several physical and chemical steps must take place for reaction to occur:9 (1) hydrogen mass transfer from the gas to the liquid phase; (2) lactic acid and hydrogen mass transfer from the liquid phase to the solid catalyst surface; (3) diffusion of hydrogen and lactic acid within the porous catalyst; and (4) conversion of lactic acid to propylene glycol via a sequence of surface chemical reaction steps. A schematic of the mass-transfer resistances present in aqueous-phase lactic acid hydrogenation is given in Figure 1. Correlations in the literature have been developed to calculate the mass-transfer coefficients across the phase boundaries. The rates of mass transfer across the phase boundaries are given in terms of masstransfer coefficients by the following expressions:
gas-liquid transfer: -RG,H2L ) kGLa(CH2* - CH2,L)
(1)
* To whom correspondence should be addressed. E-mail:
[email protected]. Voice: (517) 353-3928. † Department of Chemical Engineering. ‡ Department of Chemistry.
Figure 1. Schematic of mass transfer in a three-phase reaction.
liquid-solid transfer: -RG,LAL ) kLS,LAap(CLA,L - CLA,s)
(2)
-RG,H2L ) kLS,H2ap(CH2,L - CH2,s)
(3)
where for each reactant “j” (-RG,j) is the observed reaction rate, Cj,L and Cj,s are the bulk liquid and surface concentrations, CH2* is the aqueous-phase hydrogen solubility at the hydrogen partial pressure of the experiment, and L is the ratio of catalyst surface area to volume, equal to Dp/6 for spherical particles. Within the catalyst pellet, the influence of diffusion resistance on each reactant can be characterized by the Weisz-Prater criterion via the observable modulus:
ηφ2 )
-RGL2 CsDe
(4)
The effective diffusivity De,j ) 2Dj, where is the support porosity (0.6 for the catalyst used) and Dj is the diffusivity of reactant in liquid water estimated using the Wilke-Chang equation. If ηφ2 < 0.1 for both hydrogen and lactic acid, mass-transport resistances within the catalyst pellet can be neglected. Taking the ratio of the observable moduli for lactic acid and hydrogen gives the expression for identifying the limiting reactant within the pellet.9
ηφH22 ηφLA
2
)
1 Cs,LADe,LA b Cs,H2De,H2
10.1021/ie0104767 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/29/2002
(5)
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The value of b, the stoichiometric ratio of hydrogen to lactic acid, is 2.0 for PG formation but is actually closer to 2.3 in our studies because of side reactions to gases. If eq 5 is >1, hydrogen is the limiting reactant. Clearly, both diffusivity and solubility (e.g., liquid-phase concentration) of hydrogen play an important role in defining mass-transfer resistances. For this reason, the measurement of hydrogen solubility in lactic acid solution is included as part of this investigation. II. Experimental Section A complete description of the experimental equipment and methods is given in the dissertation of Zhang.10 Materials. Reagent-grade D,L-lactic acid (80%, J. T. Baker) in water was used in all experiments, and ultrahigh-purity hydrogen gas (99.999%, AGA) was used without further purification. The catalyst used was a 5 wt % ruthenium on activated carbon in powder form from PMC, Inc., with a mean particle diameter of 150 µm. The catalyst came as a 50 wt % slurry in water; catalyst loadings and rates are reported in the paper on a dry catalyst basis. The carbon has a N2 BET surface area of 860 m2/g, a particle porosity of 0.6, and a dry particle density of 800 kg/m3. The Ru metal dispersion before reaction as measured by hydrogen chemisorption in a Micromeritics Chemisorb 2700 is 13%. Reaction. All reactions were conducted using a 300mL stirred batch autoclave (Model 4506, Parr Instrument Co.) equipped with a gas-sparging stirrer, temperature controller, and ports for gas and liquid sampling. In a typical reaction, the catalyst (0.5-1.5-g dry basis) was first loaded into the reactor and reduced under 10% hydrogen in helium at 250 °C for 4 h. The temperature was then reduced to the desired reaction temperature, and 100 mL of lactic acid feed solution (0.56 M (5 wt %) or 1.15 M (10 wt %)) was added to the vessel. The stirring speed was set to 1200 rpm, a rate high enough to facilitate efficient operation of the gas-sparging stirrer and avoid mass-transport limitations. Temperature was then allowed to restabilize, and hydrogen pressure was increased to the desired value to initiate the reaction. Analytical. Liquid-phase samples were taken from the reactor every 30 min and were analyzed on a Spectra Tech P1000 HPLC (Thermo Separation Products) with RI detection. The column was a Biorad Aminex HPX87H at 50 °C and the mobile phase was 0.005 M H2SO4 flowing at 1 mL/min. An Ametek Dycor M100M Quadrupole Mass Spectrometer was used to continuously analyze effluent gas, which was withdrawn from the headspace above the reactor via a 2-m microcapillary quartz tube. Product gas concentrations were obtained by passing a mixed gas-calibration standard past the quartz microcapillary tube. Hydrogen Solubility. Hydrogen solubility in water and lactic acid solution was measured by a volumetric technique.10 One hundred milliliters of solution or pure water was heated in the Parr autoclave to the desired temperature, and then hydrogen was added to the autoclave to the desired pressure. After stirring to allow equilibration, 10-20 mL of liquid phase was removed from the reactor via the sampling tube into a graduated glass sample cylinder at atmospheric pressure. Dissolved hydrogen flashed out of the liquid upon reduction of pressure and was collected in a graduated cylinder submerged in a water bath. Both gas and liquid volumes were collected and the extent of hydrogen solubility was then calculated.
Figure 2. Solubility of hydrogen in water and in 10 wt % lactic acid solution. In water: (- - -) 373 K (literature11); (-) 423 K (literature11); (9) 373 K; (b) 423 K. In 10 wt % (1.15 M) lactic acid solution: (0) 373 K; (4) 403 K; (O) 423 K; (]) 443 K.
Gas-Liquid Mass-Transfer Coefficients. The gasliquid mass-transfer coefficient of hydrogen (kGLa) was measured by rapidly introducing hydrogen into the partially filled Parr autoclave and recording the transient pressure decay. Typically, the reactor was filled to 75-90% of liquid capacity to reduce headspace sufficiently to achieve a significant decrease in hydrogen pressure. Experiments were conducted at 298 K and at different stirring rates. The mass-transfer coefficient was calculated by integrating the time-dependent differential equation describing gas pressure change as mass transfer proceeds across the gas-liquid-phase boundary. III. Results and Discussion Hydrogen Solubility. The measured solubility of hydrogen in water and in 10 wt % (1.15 M) lactic acid solution at several temperatures is given in Figure 2. Hydrogen solubility in water as reported in the literature11 is also shown in Figure 2. The experimental data in water match well with the data from the literature, thus verifying the reliability of the measurements. In 10% lactic acid solution, the solubility of hydrogen has the same trend in temperature as in water, but the solubility in lactic acid solution is about 10% lower than that in pure water at the same temperature. This decline in solubility with addition of an organic species to water is observed in numerous other aqueous-phase systems.11 These data (as CH2*) were used in the calculation of mass-transfer rates in the following analysis. Reaction Rate Data. A summary of the reaction rate data collected is given in Figures 3 and 4. Reactions were conducted at 403 and 423 K at pressures ranging from 6.8 to 13.6 MPa H2, with a lactic acid feed concentration of 10 wt % (1.13 M) and at catalyst loadings ranging from 0.5 to 1.5 g (dry basis)/100 g of solution. The selectivity to propylene glycol ranged from 75 to 95%, with methane, ethane, and propane being the primary side products. Different catalyst loadings were used as a direct test of the influence of mass transport on the reaction.9 The reaction rate was calculated from Figures 3 and 4 via differential analysis. Lactic acid conversion vs time was fit to a fourth-order polynomial, which was then differentiated and evaluated at various times over the
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Figure 3. Lactic acid conversion vs reaction time at 403 K. (Starting material: 100 g of 10 wt % (1.15 M) lactic acid in water). Catalyst charge: (9) 0.5 g; (2) 1.0 g; (b) 1.5 g. Hydrogen pressure: (- - -) 6.8 MPa; (-) 10.2 MPa; (- -) 13.6 MPa.
Figure 4. Lactic acid conversion vs reaction time at 423 K. (Starting material: 100 g of 10 wt % (1.15 M) lactic acid in water). Catalyst charge: (9) 0.5 g; (2) 1.0 g; (b) 1.5 g. Hydrogen pressure: (- - -) 6.8 MPa; (-) 10.2 MPa; (- -) 13.6 MPa.
course of reaction. By incorporation of lactic acid concentration and the quantity and properties of catalyst, reaction rate on a unit catalyst volume basis (kmol of lactic acid/m3 of catalyst/s) or a unit fluid volume basis (kmol/m3/s) versus lactic acid concentration is determined. Mass-Transfer Analysis. The gas-liquid masstransfer coefficient for hydrogen, kGLa, was measured experimentally by transient hydrogen absorption as described earlier. The value of kGLa was also estimated using the correlations of Bern et al.12 and of Yagi and Yoshida,13 using estimated properties of the reaction fluid and the Ru/C catalyst. The superficial gas velocity in the reactor was taken as ug ) 0.1 cm/s, a conservative value for a reactor equipped with gas sparging. Experimental data collected by Choudhary et al.14 were also included in the comparison. Experimental results and literature values are given in Figure 5; the measured values of the mass-transfer coefficient agree reasonably well with the data of Choudhary et al. and with Bern’s correlation. The correlation of Yagi and Yoshida gave mass-transfer coefficients that were 6-8 times larger than those measured or predicted by Bern; these values are not included on the graph. The liquid-solid mass-transfer coefficient kLS,j was calculated using the correlation of Boon-Long et al.15 and the equation of Brian et al.16 The mass-transfer area ap is the external area of catalyst particles per unit volume reaction fluid, estimated using catalyst loading
Figure 5. Mass-transfer coefficients. Gas-liquid: (b) experimental data (this work); (9) data of Choudhary et al.;14 (- - -) correlation of Bern et al.12 Liquid-solid: (-) correlation of Boon-Long et al.15
and properties given earlier in this paper. The dependence of kLS,jap on the stirring rate as described by the Boon-Long correlation, the more conservative of the two relationships, is shown in Figure 5; these values were used in the calculations. Intraparticle mass transport is described via the observable modulus ηφ2 (eq 4), with L ) Dp/6 ) 25 µm and surface concentrations Cj,s calculated from eqs 1-3. The relative importance of each mass-transfer resistance can be ascertained by calculating the maximum mass-transfer rate across the phase boundary and comparing it to the observed reaction rate of the experiment. This maximum mass-transfer rate is the product of the mass-transfer coefficient and maximum species concentration at reaction conditions (e.g., -RGL,max ) kGLaC*H2). If the maximum mass-transfer rate is much larger than the observed reaction rate, then mass transfer does not limit reaction across the specified phase boundary. Analysis of mass-transport resistances has been done for several of the experiments reported in Figures 3 and 4. All evaluations were done at the onset of the reaction, where the actual reaction rate and thus the influence of mass-transfer resistances are the greatest. A summary of mass-transfer calculations for the experiment at 423 K, 13.6 MPa H2 pressure, and 1.5 g of catalyst, at which rate is highest and thus mass transfer is most likely to influence the reaction, is given in Table 1. Both mass transfer and reaction rates in Table l are reported on a per unit fluid volume basis. Comparison of maximum mass-transfer rates with reaction rate shows that liquid-solid mass-transfer resistances exert no influence on reaction. The values of observable modulus for both hydrogen and lactic acid are substantially lower than 0.1, so intraparticle masstransfer resistances can also be neglected. This is primarily because very small particles (0.15 mm) were used in the investigation. For the experiment analyzed in Table 1, gas-liquid mass transfer reduces the liquid-phase surface hydrogen concentration (CH2,s) by about 5% relative to the solubility limit (C*H2). This indicates that gas-liquid mass transport has a minor effect on the overall reaction rate under these conditions. This conclusion is further supported by experiments at different catalyst loadings in which we observe that reaction rate per unit catalyst mass is essentially constant for three catalyst loadings ranging from 0.5 to 1.5 g/100 g of solution.
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Table 1. Results of Mass-Transfer Analysisa mass-transfer coefficient (units) gas-liquid mass transfer liquid-solid mass transfer intraparticle mass transfer observed reaction rate a
kGLa (s-1) kLS,H2 (m/s) kLS,LA (m/s) aP (m-1) ηφH22 ηφLA2 -RG, H2 -RG,LA
value of coefficient 0.083 1.42 × 10-3 8.5 × 10-4 750 0.015 0.002
maximum mass-transfer rate or reaction rate (fluid basis) (kmol/m3/s) 0.01 0.13 0.70
reference Bern et al.12 Boon-Long et al.14
5.1 × 10-4 2.5 × 10-4
Conditions: 423 K, 13.6 MPa H2, 1.5 g of cat/100 g of solution, 1.1 M LA in H2O, 1200 rpm.
We conducted a series of experiments at stirring rates ranging from 400 to 1200 rpm to further investigate the influence of gas-liquid mass transfer on rate. We observed no measurable dependence of reaction rate on stirring speed over this range, even though evaluation of gas-liquid mass transport using experimental kGLa values in Figure 5 shows that liquid-phase surface hydrogen concentration (CH2,s) is reduced from the solubility limit (C*H2) by as much as 40% at 400 rpm. However, this reduction in concentration only occurs at the initial, maximum reaction rate; as the reaction proceeds, the reaction rate declines and mass transfer has less effect. Also, the dependence of reaction rate on hydrogen concentration or partial pressure is weak according to the kinetic model (developed below), so any reduction in hydrogen concentration does not greatly affect rate. These two factors together result in no observable influence of stirring speed on reaction rate over the course of lactic acid conversion. Initial Rate Kinetics. Because mass transfer does not significantly influence the rate of lactic acid hydrogenation at 1200 rpm, the conversion rate profiles given in Figures 3 and 4 represent intrinsic reaction kinetics for lactic acid to PG. Further, because the reaction is mildly exothermic (∆H ) -43 kJ/mol) and highly favored thermodynamically (equilibrium conversion > 0.98 at reaction conditions), temperature change during reaction can be neglected and the reaction can be assumed to be irreversible for practical purposes. Initial rates were calculated by applying the differential method described earlier and calculating the rate at t ) 0. These rates were fit to a simple nth order expression to obtain an estimate of activation energy and dependence on hydrogen pressure. This initial rate evaluation also avoids possible complications from catalyst deactivation. The resulting rate expression is given in eq 6 with PH2 in MPa and T in K. The apparent
-RiG,LA (kmol/kg of cat/s) ) 1.4 × 106 exp(-11310/T)P0.23 H2 (6) activation energy of the initial reaction from this rate expression is 94 kJ/mol, a value consistent with a chemically controlled rate process. Langmuir-Hinshelwood Kinetics. Even though lactic acid hydrogenation is relatively simple in that there are few byproducts, the precise steps of the catalytic surface reaction are as yet unknown. (Insight into the mechanism of the reaction is ongoing in our laboratory.17) A simple Langmuir-Hinshelwood model in which molecular hydrogen and lactic adsorb on the catalyst surface and subsequently react to form PG is
Figure 6. Effect of propylene glycol addition on lactic acid reaction rate. (423 K, 10.2 MPa H2, 10 wt % lactic acid solution, 1.0 g of cat/100 g of solution). Propylene glycol concentration: ([) 0.0; (9) 0.72 M; (b) 1.4 M; (2) 2.9 M.
thus implemented as a conceptual description of the reaction.
(1) LA + S ) LA‚S (fast)
(7a)
(2) H2 + S ) H2‚S (fast)
(7b)
(3) H2‚S + LA‚S f P1‚S + S (slow)
(7c)
(4) P1‚S + H2‚S ) PG‚S (fast)
(7d)
(5) PG‚S ) PG + S (fast)
(7e)
The rate-limiting step is considered irreversible, and the adsorption of water is neglected. The total catalyst site density CT is considered constant, and all sites are considered equivalent. Several variations of the above L-H model were evaluated, including those with dissociative hydrogen adsorption or adsorption of only one species (lactic acid or hydrogen), but all of them gave negative rate or adsorption constants. Only the above model involving molecular adsorption of lactic acid and hydrogen, surface reaction of lactic acid with hydrogen as the rate-limiting step, and finally desorption of PG gave all positive kinetic and adsorption constants with the correct dependence on temperature. To investigate the influence of PG on reaction kinetics, we conducted several experiments in which PG was added to lactic acid in varying concentrations. The results are given in Figure 6 for PG concentrations up to about 2.5 times that of the lactic acid feed concentration; it is clear that the presence of PG has little effect on the lactic acid hydrogenation rate. Therefore, the PG term in the denominator of the L-H rate equation is
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Figure 7. Parity plot for Langmuir-Hinshelwood kinetic expression at 423 K. Table 2. Rate Constants for L-H Rate Expression (Equation 8) constant (m3/kg
k1 of cat/MPa/s) KH2 (MPa1-) KLA (m3/kmol) R2
403 K 10-6
1.47 × 3.7 × 10-3 0.55 0.73
423 K 1.75 × 10-6 1.2 × 10-3 0.28 0.86
eliminated. The final form of the L-H kinetic expression based on the above model is thus given as
-RG,LA (kmol/kg of cat/s) )
kCLAPH2 (1 + KH2PH2 + KLACLA)2 (8)
The experimental data at 403 and 423 K (Figures 3 and 4) were fit to the rate expression using MathCad. For each experiment, the rate was calculated by differentiation as described above at 10 points over the course of reaction, thus giving a total of 90 concentration-rate points at 403 K and 80 points at 423 K. (The experiment at 0.5 g of catalyst loading and 6.8 MPa at 423 K was eliminated from the regression because it did not follow the trends of the other experiments and upon being fit fell far outside the range of the rest of the data.) The fitted rate constants at each temperature and the values of the regression coefficient R2 are given in Table 2. A parity plot illustrating the agreement between experimental and predicted rates at 423 K is shown in Figure 7. (The parity plot at 403 K is similar.) The fit of the data is good at both temperatures, considering the simplifying assumptions made in the L-H kinetic model and the presence of side reactions that consume 5-10% of the lactic acid in the reactor as byproduct gases. This gas formation, the extent of which depends on temperature and hydrogen pressure, consumes significant quantities of hydrogen and is responsible for some of the scatter in the data. Using only initial rate data to fit the L-H expression gave similar regression coefficients, an indication that catalyst deactivation is not a major factor in batch studies. The estimated heats of adsorption of lactic acid and hydrogen, based on the adsorption constants KLA and
KH2 at the two temperatures (Ki ) Ko,i exp(∆Hi/RT)), are ∆HLA ) 47 kJ/mol and ∆HH2 ) 79 kJ/mol. These are reasonable values for chemisorbed species. The activation energy for the surface reaction rate constant ks (eq 7c) is determined from the composite L-H rate constant k1 ) ksKLAKH2Ct2 and is equal to 138 kJ/mol, again a reasonable value for a surface reaction step. The reaction rate -RG,LA (kmol/kg of cat/s) given in eq 8 can be converted into turnover frequency TOF (mol of LA/mol of surface Ru/s) using the measured Ru loading of 5 wt % and Ru dispersion of 13%. The relationship is given as TOF ) 1.55 × 104 (-RG,LA). The maximum turnover frequency from any of our experiments is 0.26 s-1. In a recent review on liquid-phase catalytic hydrogenation, Singh and Vannice18 postulate that under some conditions molecular hydrogen is the appropriate species to consider in liquid-phase hydrogenation kinetics. This assumption is viable provided dissociative adsorption is relatively weak so that the formation of a molecular hydrogen “transition state” on the catalyst surface limits the overall rate of hydrogen adsorption. The use of molecular hydrogen instead of dissociated hydrogen as the adsorbed species at the catalyst surface is thus justified. Further, the fit of experimental data was clearly superior in our case when molecular hydrogen was considered the adsorbing species instead of dissociatively adsorbed hydrogen. The L-H rate expression provides insight into the relative surface concentrations of hydrogen and lactic acid on the catalyst, as according to the L-H model the denominator terms give the relative concentrations of vacant, hydrogen-occupied, and lactic-acid-occupied surface sites. At the highest hydrogen pressure (13.6 MPa) at 403 K and initial conditions (CLA ) 1.13 M), the ratio of empty (1), hydrogen-occupied (KH2PH2), and lacticacid-occupied (KLACLA) sites is 1:0.05:0.6. Thus, the catalyst surface has significant lactic acid adsorbed upon it but relatively little hydrogen. This is not surprising in light of the much lower concentration and higher stoichiometric requirement for hydrogen than for lactic acid and is consistent with the experimental evidence
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that increasing hydrogen pressure increases the reaction rate. Our mechanistic investigations17 using isotopic (HD) labeling and computational molecular modeling suggest that lactic acid hydrogenation takes place through one of two mechanistic pathways involving three or four specific surface reaction steps. The rigorous rate expressions based on these proposed mechanisms are significantly more complex and have several kinetic constants in addition to those of the L-H model developed here. While additional adjustable constants offer the possibility of a better fit of the data, at this point the implementation of a more rigorous, mechanistically based rate expression is premature. The L-H model presented here, with a single surface reaction of lactic acid and hydrogen as the rate-limiting step, is a simple and useful model for design purposes. IV. Conclusions Calculations based on experiments and literature correlations show that gas-liquid, liquid-solid, and intraparticle mass-transfer resistances can be neglected in the batch hydrogenation of lactic acid to PG at the conditions implemented. The intrinsic reaction rate data at 403 and 423 K were fit to a Langmuir-Hinshelwoodtype rate expression. The fit of the data was good considering the existence of side reactions and the range of reaction parameters implemented. The kinetic model serves as a useful tool for further research and design of a process to produce propylene glycol from fermentation-derived lactic acid. Literature Cited (1) Zhang, Z.; Jackson, J. E.; Miller, D. J. Aqueous-Phase Hydrogenation of Lactic Acid to Propylene Glycol. Appl. Catal. A 2001, 219, 89. (2) Bowden, E.; Adkins, H. Hydrogenation of Optically Active Compounds over Nickel and Copper-Chromium Oxide. J. Am. Chem. Soc. 1934, 56, 689. (3) Adkins, H.; Pavlic, A. Hydrogenation of Esters to Alcohols over Raney Nickel. I. J. Am. Chem. Soc. 1947, 69, 3039.
(4) Adkins, H.; Billica, H. The Hydrogenation of Esters to Alcohols at 25-150 °C. J. Am. Chem. Soc. 1948, 70, 3121. (5) Broadbent, H.; Campbell, G.; Bartley, W.; Johnson, J. Ruthenium and Its Compounds as Hydrogenation Catalysts. J. Org. Chem. 1959, 24, 1847. (6) Carnahan, B.; Ford, T.; Gresham, W.; Grisby, W.; Hager, G. Ruthenium-Catalyzed Hydrogenation of Acids to Alcohols. J. Am. Chem. Soc. 1955, 77, 3766. (7) Antons, S.; Beitzke, B. Process for Preparing Optically Active Amino Alcohols. U.S. Patent 5,536,879, 1996. (8) Antons, S. Process for the Preparation of Optically Active Alcohols. U.S. Patent 5,731,479, 1998. (9) Ramachandran, P.; Chaudhari, R. Three-Phase Catalytic Reactors; Gordon and Breach Science Publishers: Philadelphia, PA, 1983. (10) Zhang, Z. Aqueous-Phase Hydrogenation of BiomassDerived Lactic Acid to Propylene Glycol. Ph.D. Dissertation, Michigan State University, East Lansing, MI, 2000. (11) Stephen, H.; Stephen, T. The Solubility of Inorganic and Organic Compounds, Vol. 1; MacMillan: New York, 1963. (12) Bern, L.; Lidefelt, J.; Schoon, N. Mass Transfer and Scaleup in Fat Hydrogenation. J. Am. Oil Chem. Soc. 1976, 53, 463. (13) Yagi, H.; Yoshida, F. Gas Absorption by Newtonian and Non-Newtonian Fluids in Sparged Agitated Vessels. Ind. Eng. Chem. Process Des. Dev., 1975, 14, 488. (14) Choudhary, V.; Sane, M.; Tambe, S. Kinetics of Hydrogenation of o-Nitrophenol to o-Aminophenol on Pd/Carbon Catalysts in a Stirred Three-Phase Slurry Reactor. Ind. Eng. Chem. Res. 1998, 37, 3879. (15) Boon-Long, S.; Laguerie, C.; Couderc, J. Mass Transfer from Suspended Solids to a Liquid in Agitated Vessels. Chem. Eng. Sci. 1978, 33, 813. (16) Brian, P.; Hales, H.; Sherwood, T. Transport of Heat and Mass between Liquids and Spherical Particles in an Agitated Tank. AIChE J. 1969, 15, 419. (17) Kovacs, D.; Miller, D.; Jackson, J. Mechanistic Investigation of Lactic Acid Hydrogenation. ACS Abstracts, 2001 Spring Annual Meeting, San Diego, CA, April 2001. (18) Singh, U.; Vannice, A. Kinetics of Liquid-Phase Hydrogenation Reactions over Supported Metal CatalystssA Review. Appl. Catal. A 2001, 213, 1.
Received for review May 30, 2001 Revised manuscript received October 24, 2001 Accepted November 21, 2001 IE0104767
